mountain rice vs ricegrass

mountain rice and ricegrass both are nouns.

Word	Noun	Adjective	Verb	Adverb
mountain rice	Yes	No	No	No
ricegrass	Yes	No	No	No

As nouns, ricegrass is a hypernym of mountain rice; that is, ricegrass is a word with a broader meaning than mountain rice:

mountain rice: valuable forage grass of dry upland areas and plains of western North America to northern Mexico
ricegrass: any grass of the genus Oryzopsis

Other hypernyms of mountain rice include rice grass.

mountain rice (noun)	ricegrass (noun)
valuable forage grass of dry upland areas and plains of western North America to northern Mexico	any grass of the genus Oryzopsis